In recent years, light-emitting elements using light-emissive organic compounds have actively been researched and developed. Such light-emitting elements basically have a structure where a layer containing a light-emissive organic compound is sandwiched between a pair of electrodes. By applying a voltage to such elements, electrons and holes are respectively injected into the layer containing a light-emissive organic compound from the pair of electrodes, and thus current flows therein. Then, these carriers (electrons and holes) are recombined with each other to bring the light-emissive organic compound into an excited state. The light-emissive organic compound emits light upon returning from the excited state to the ground state. Because of such a mechanism, the light-emitting element is called a light-emitting element of a current excitation type.
Note that as the kind of excited states formed by an organic compound, there are a singlet excited state and a triplet excited state. Light emission obtained from the singlet excited state is called fluorescence, while light emission obtained from the triplet excited state is called phosphorescence.
Since such a light-emitting element is formed of an organic thin film with a thickness of about, for example, 0.1 μm, it has a big advantage in that manufacture with thin shape and light weight is enabled. In addition, there is another advantage in that quite a high response speed is achieved since it takes only about 1μ second or less from the time when carriers are injected until the light emission is obtained. Such characteristics are considered as advantageous for a flat panel display element.
Further, since such a light-emitting element is formed in a film form, light emission with a plane surface can be easily obtained by forming a light-emitting element with a large area. Such an advantage cannot be obtained easily with either a point light source typified by an incandescent lamp or an LED or a linear light source typified by a fluorescent lamp; therefore, the light-emitting element has a high potential to be used as a surface light source which can be applied to illumination or the like.
As described above, a light-emitting element of a current excitation type which uses a light-emissive organic compound is expected to be applied to various fields such as a light-emitting device and illumination; however, there remain a number of essential tasks to be completed. As one of the tasks, there is a reduction in power consumption. In order to reduce power consumption, it is required to reduce the driving voltage of the light-emitting element. Since the emission intensity of a light-emitting element of a current excitation type is determined by the amount of current flowing therein, it is required to flow large current with low voltage in order to reduce the driving voltage of the light-emitting element.
According to Patent Document 1, it is reported that a driving voltage of a light-emitting element can be decreased by doping an organic layer which is in contact with an anode with an electron-accepting compound having a property capable of oxidizing an organic compound contained in the organic layer. The organic layer doped with an electron-accepting dopant does not cause a voltage rise in the element even when it is formed thick; therefore, a distance between the electrodes can be set longer than usual, which is advantageous in that a possibility of causing a short circuit can be significantly reduced.
Meanwhile, various attempts have been made to increase the extraction efficiency of light emitted from a light-emitting element to the outside by adjusting the optical distance. According to Patent Document 2, it is reported that the emission spectrum can be controlled by changing the thickness of an organic compound layer doped with an electron-accepting compound which is provided in the interface between an anode and a light-emitting layer.
The organic compound layer doped with an electron-accepting compound disclosed in Patent Document 1 and Patent Document 2 is formed by adding an electron-accepting compound by co-deposition or by applying a liquid solution which is adjusted by reacting an electron-accepting compound with an organic compound. Therefore, the organic compound and the electron-accepting compound are uniformly mixed, and thus the organic compound layer doped with the electron-accepting compound has isotropic conductivity.
Accordingly, in order to control the electrical characteristics such as conductivity, it has been required to change the composition ratio of the organic compound to the electron-accepting compound included in the layer, or to change the kind of compounds included in the layer.
However, when the composition ratio of the layer is changed or the kind of compounds included in the layer is changed, characteristics other than the electrical characteristics, for example, optical characteristics (e.g., refractive index) change. If the optical characteristics change, the emission spectrum changes, which in turn changes the emission color and the extraction efficiency of light to the outside. Thus, it has been difficult to change the electrical characteristics without changing the optical characteristics.
That is, light-emitting elements are required to be designed exactly in consideration of various characteristics such as the electrical characteristics and optical characteristics. In addition, light-emitting elements have been required to be manufactured very exactly in accordance with its original design.
[Patent Document 1]
Japanese Patent Laid-Open No. H11-251067
[Patent Document 2]
Japanese Patent Laid-Open No. 2001-244079